linux-kernel - Re: [PATCH] KVM: x86/svm/pmu: Set PerfMonV2 global control bits correctly

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Message-ID: <f5bbe9ac-ca35-4c3e-8cd7-249839fbb8b8@linux.intel.com>
Date: Mon, 4 Mar 2024 15:59:05 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sandipan Das <sandipan.das@....com>, Like Xu <like.xu.linux@...il.com>
Cc: seanjc@...gle.com, pbonzini@...hat.com, mizhang@...gle.com,
 jmattson@...gle.com, ravi.bangoria@....com, nikunj.dadhania@....com,
 santosh.shukla@....com, manali.shukla@....com, babu.moger@....com,
 kvm list <kvm@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/svm/pmu: Set PerfMonV2 global control bits
 correctly


On 3/1/2024 5:00 PM, Sandipan Das wrote:
> On 3/1/2024 2:07 PM, Like Xu wrote:
>> On 1/3/2024 3:50 pm, Sandipan Das wrote:
>>> With PerfMonV2, a performance monitoring counter will start operating
>>> only when both the PERF_CTLx enable bit as well as the corresponding
>>> PerfCntrGlobalCtl enable bit are set.
>>>
>>> When the PerfMonV2 CPUID feature bit (leaf 0x80000022 EAX bit 0) is set
>>> for a guest but the guest kernel does not support PerfMonV2 (such as
>>> kernels older than v5.19), the guest counters do not count since the
>>> PerfCntrGlobalCtl MSR is initialized to zero and the guest kernel never
>>> writes to it.
>> If the vcpu has the PerfMonV2 feature, it should not work the way legacy
>> PMU does. Users need to use the new driver to operate the new hardware,
>> don't they ? One practical approach is that the hypervisor should not set
>> the PerfMonV2 bit for this unpatched 'v5.19' guest.
>>
> My understanding is that the legacy method of managing the counters should
> still work because the enable bits in PerfCntrGlobalCtl are expected to be
> set. The AMD PPR does mention that the PerfCntrEn bitfield of PerfCntrGlobalCtl
> is set to 0x3f after a system reset. That way, the guest kernel can use either


If so, please add the PPR description here as comments.


> the new or legacy method.
>
>>> This is not observed on bare-metal as the default value of the
>>> PerfCntrGlobalCtl MSR after a reset is 0x3f (assuming there are six
>>> counters) and the counters can still be operated by using the enable
>>> bit in the PERF_CTLx MSRs. Replicate the same behaviour in guests for
>>> compatibility with older kernels.
>>>
>>> Before:
>>>
>>>     $ perf stat -e cycles:u true
>>>
>>>      Performance counter stats for 'true':
>>>
>>>                      0      cycles:u
>>>
>>>            0.001074773 seconds time elapsed
>>>
>>>            0.001169000 seconds user
>>>            0.000000000 seconds sys
>>>
>>> After:
>>>
>>>     $ perf stat -e cycles:u true
>>>
>>>      Performance counter stats for 'true':
>>>
>>>                227,850      cycles:u
>>>
>>>            0.037770758 seconds time elapsed
>>>
>>>            0.000000000 seconds user
>>>            0.037886000 seconds sys
>>>
>>> Reported-by: Babu Moger <babu.moger@....com>
>>> Fixes: 4a2771895ca6 ("KVM: x86/svm/pmu: Add AMD PerfMonV2 support")
>>> Signed-off-by: Sandipan Das <sandipan.das@....com>
>>> ---
>>>    arch/x86/kvm/svm/pmu.c | 1 +
>>>    1 file changed, 1 insertion(+)
>>>
>>> diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
>>> index b6a7ad4d6914..14709c564d6a 100644
>>> --- a/arch/x86/kvm/svm/pmu.c
>>> +++ b/arch/x86/kvm/svm/pmu.c
>>> @@ -205,6 +205,7 @@ static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
>>>        if (pmu->version > 1) {
>>>            pmu->global_ctrl_mask = ~((1ull << pmu->nr_arch_gp_counters) - 1);
>>>            pmu->global_status_mask = pmu->global_ctrl_mask;
>>> +        pmu->global_ctrl = ~pmu->global_ctrl_mask;


It seems to be more easily understand to calculate global_ctrl firstly 
and then derive the globol_ctrl_mask (negative logic).

diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
index e886300f0f97..7ac9b080aba6 100644
--- a/arch/x86/kvm/svm/pmu.c
+++ b/arch/x86/kvm/svm/pmu.c
@@ -199,7 +199,8 @@ static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
kvm_pmu_cap.num_counters_gp);

         if (pmu->version > 1) {
-               pmu->global_ctrl_mask = ~((1ull << 
pmu->nr_arch_gp_counters) - 1);
+               pmu->global_ctrl = (1ull << pmu->nr_arch_gp_counters) - 1;
+               pmu->global_ctrl_mask = ~pmu->global_ctrl;
                 pmu->global_status_mask = pmu->global_ctrl_mask;
         }

>>>        }
>>>          pmu->counter_bitmask[KVM_PMC_GP] = ((u64)1 << 48) - 1;
>